Mercedes NAG1 / WA580 / W5A580 automatic transmission

by Michael Dickens

The NAG1 (WA580 or W5A580) automatic transmission found in many Chrysler products is an electronically controlled 5-speed transmission system with a lock-up clutch in the torque converter. The fifth gear is designed to be an overdrive with a high-speed ratio; the ratios for the gear stages are obtained by three planetary gear sets.

The reasons why Chrysler switched to this transmission are somewhat unclear; the decision was made shortly after the acquisition of a highly profitable Chrysler Corporation by Daimler-Benz, and many alterations appeared to have been made for the benefit of Mercedes to the detriment of Chrysler. That said, the W5A580 / WA580 transmission did have advantages over existing Chrysler automatics. It was more efficient than the four-speed automatic used with lesser LX-car engines at launch, and compared with Chrysler’s own five-speed rear-drive automatic, the 545RFE, it had a wider gear range, considerably smaller size, and lower weight. The 545RFE remained in use alongside the WA580, in applications where its size was not an issue.

The term NAG1 is used to identify a family of transmissions, including the W5A580 / WA580 model (the only difference is the name, which is applied inconsistently) used by Chrysler. The meaning of these acronyms is:

N = New

A = Automatic

G = Gearbox

1 = Generation 1

W = Transmission using a hydraulic torque converter.

5 = 5 forward gears [not always written out].

A = Automatic transmission.

580 = Maximum input torque capacity in Newton-meters (428 ft-lbs.)

The NAG1 gears are actuated hydraulically, using fully electronic controls, using combinations of three multi-disc holding clutches, three multi-disc driving clutches, and two freewheeling clutches. Electronic control, pioneered by Chrysler and Toyota (among others), can increase gas mileage, increase shift control, provide a more favorable step-up through each of the five gears; increase service life, and lower maintenance costs.

The WA580 was used in many different Chrysler products, starting with the 2005 Chrysler 300 and Dodge Magnum; it was used with the 3.5 liter V6 and Hemi V8 engines. Later, it was used in the Jeep Liberty, Grand Cherokee, Dodge Charger, Dodge Challenger, and Ram truck, as well as every automatic-transmission SRT8 through at least 2012, and the 2012 Jeep Wrangler. The transmission was first launched in 2003 by Mercedes; one engineer familiar with it believes it to have been a copy of the five-speed ZF automatic.

Visually, the NAG1 transmission can usually be identified by a round 13-way connector near the front corner of the transmission oil pan, on the right side. Specific transmission information is usually found stamped into a pad that is located on the left side of the transmission, above the oil pan rail. The gear ratios are:

Gear

WA580

545RFE

1

3.59

3.00

2 / 2-kickdown

2.19

1.67 / 1.50

3

1.41

1.00

4

1.00

0.75

5

0.83

0.67

Reverse

3.2*

3.00

* Different specification listings noted both 3.16 and 3.17. 3.16 appears more likely with 3.17 likely to be a typo. It is possible that the actual ratio is 3.165 and it has been rounded up or down, alternately.

Technical details

Chrysler built the W5A580 at its Indiana Transmission Plant II, on the north side of Kokomo. The plant had 6 million square feet of space and 695 employees; it was completed in 2003 and was created to build this transmission. Indiana Plant I built the 45RFE and 545RFE electronically-controlled transmissions starting in 1998.

The converter and transmission housings are made from a light alloy; they are bolted together and sealed by a coated intermediate plate. The oil pump and the outer disc carrier are bolted to the converter housing. The stator shaft is pressed into it and prevented from rotating by splines. The control unit is bolted to the transmission housing from underneath and closed in by a sheet metal steel oil pan.

The mechanical section consists of a input shaft, output shaft, a sun gear shaft, and three planetary gear sets which are coupled to each other. The planetary gear sets each have four planetary pinion gears. The oil pressure for the torque converter lock-up clutch and internal clutches is supplied through bores in the input and output shaft. All the bearing points of the gear sets, as well as the freewheeling clutches and actuators, are supplied with oil. The parking lock gear is connected to the output shaft via splines.

The control unit is made from the shift plate for the hydraulic control, and an electrical control unit contained in a plastic box which is screwed to the shift plate. Strip conductors make the connection between the electrical components and a 13-pin bayonet-lock plug connector, which makes the connection to the vehicle’s transmission computer.

Hydraulic components, including actuators, are responsible for pressure distribution. Each shift group contains a command valve, a holding pressure shift valve, a shift pressure shift valve, overlap regulating valve, and a solenoid. There are three shift groups: 1-2/4-5, 2-3, and 3-4.

Mercedes W5A580 transmission woes

Throughout the WA580’s / W5A580’s life, readers warned that the transmission was extremely
sensitive to fluid quality and would shudder badly when the torque
converter clutch is applied with even a 0.5% water dose. The temporary
fix was to flush the fluid thoroughly (Chrysler recommended a triple
flush) and put in a new filter; the long-term fix was to apply RTV
(form-a-gasket) to the base of the transmission fill tube to avoid
future contamination (particularly on early models, made in 2003-2005), assuming that's where water got in. If the fill
tube was incorrectly installed - that is, if the seal was not perfectly
flush with the transmission housing, usually above it - then that must
be fixed first, so it is completely flush. The procedure could take up to two hours and was covered in TSB
21-011-05 (it was covered by warranty at that time).

Another problem, at least through 2007, was the transmission filler tube o-ring leaking. Keith K. wrote that this was:

the “transmission cup plug adapter bushing.” The old ones had two white o-rings that were available separately; they all leaked and it looks like a pan gasket leak, there is a TSB from 2005 or 2006 about it. They superceded the O-rings to the whole bushing so you get a new plug bushing and O-rings, new black ones, already installed on it. The new design won't leak.

Paul Knisely helped a reader whose W5A580 was making a vibration and low-frequency noise in drive, with the MDS activated between 18 and 50 mph. Four different dealerships had said that the system was working normally, and suggested driving in fourth gear to avoid overdrive. Paul checked the problem with Alldata DIY and found a diagnosis that the clutch in the torque converter was not holding — showing it was, but not actually holding. This would cause the vibration.

For performance, “MoparMaven” reported hearing that one of the clutches in the WA580 transmission (not surprisingly) can't handle full power on the 4-5 shift from heavily modified engines (e.g. 700 horsepower and up). “Some clutch cannot take the rpm drop without mechanical advantage to break the tires loose, if only for an instant, because there was a declutching and reclutching. I'm guessing that the engine management system keeps the rpm from running away while this takes place... they recommend lifting for the 4-5 upshift.”

Some critics complained about the sometimes-heavy-handed torque management applied to keep the WA580 reliable, with the engine cutting power noticeably on shifts. Throughout the transmission’s life at Chrysler, critics also complained of its slow shifting. Still, as one forum poster wrote:

Chrysler may have overpaid for the NAG1 but it is hardly the slow shifting, unreliable transmission that the auto mags love to call it. Compared to a manual transmission, a human cannot shift faster than a NAG1. The NAG1 has powered some incredibly high horsepower LX vehicles with very little problems. Treat it badly and don't maintain it, and it will break like anything else. There are plenty of LX/Jeep owners who have ha absolutely no problems with the NAG1/WA580.

Early reviews of the transmission actually praised its relatively fast shifting, indicating that the “slow shift” issue may have come later, perhaps due to the torque management intended to prolong transmission life.